An early event in programmed cell death is a remodeling of plasma membrane phospholipids (PL) that results in de novo exposure of phosphatidylserine (PS) and other aminophospholipids at the cell surface. This exposure of thee aminophospholipids is known to promote activation of the plasma complement and coagulation proteases and is implicated in the recognition and clearance of apoptotic cells by the reticuloendothelial system. We recently identified a new plasma membrane protein (PL scramblase) that mediates bi-directional movement of membrane PL and our preliminary data suggest that this protein may also function in amplifying the execution phase of apoptosis. In this Proposal we aim to identify the cellular mechanism(s) responsible for transbilayer movement of plasma membrane PL during programmed cell death, and to determine how this reorganization of cell surface PL is causally related to other changes observed in the apoptotic cell. Our Specific Aims include: (1) To determine whether the egress of PS to the surface of apoptotic cells is mediated through activation of PL scramblase, or through alternative mechanism(s); (2) To identify which of the metabolic changes observed in apoptotic cells are causally linked to the topologic rearrangement of plasma membrane PL; (3) To determine whether the early reorganization of plasma membrane PL is required for the subsequent degradative processes observed during programmed cell death; and (4) To determine whether the level of expression of plasma membrane PL scramblase influences cell susceptibility to growth arrest or to phagocytosis in vitro, and affects either cell survival or cell clearance through the reticuloendothelial system in vivo. It is proposed that these experiments will yield new insight into how activation of the effector pathways of apoptosis is causally related to the transbilayer reorganization of plasma membrane phospholipids in lymphocytes and other cells.